On Wed, Mar 10, 2021 at 8:12 PM Alex Ghiti [off-list ref] wrote:

Le 3/10/21 à 6:42 AM, Arnd Bergmann a écrit :

quoted

On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti [off-list ref] wrote:

quoted

Le 2/25/21 à 5:34 AM, David Hildenbrand a écrit :

quoted

                   |            |                  |         |> +
ffffffc000000000 | -256    GB | ffffffc7ffffffff |   32 GB | kasan

quoted

+   ffffffcefee00000 | -196    GB | ffffffcefeffffff |    2 MB | fixmap
+   ffffffceff000000 | -196    GB | ffffffceffffffff |   16 MB | PCI io
+   ffffffcf00000000 | -196    GB | ffffffcfffffffff |    4 GB | vmemmap
+   ffffffd000000000 | -192    GB | ffffffdfffffffff |   64 GB |
vmalloc/ioremap space
+   ffffffe000000000 | -128    GB | ffffffff7fffffff |  126 GB |
direct mapping of all physical memory

^ So you could never ever have more than 126 GB, correct?

I assume that's nothing new.

Before this patch, the limit was 128GB, so in my sense, there is nothing
new. If ever we want to increase that limit, we'll just have to lower
PAGE_OFFSET, there is still some unused virtual addresses after kasan
for example.

Linus Walleij is looking into changing the arm32 code to have the kernel
direct map inside of the vmalloc area, which would be another place
that you could use here. It would be nice to not have too many different
ways of doing this, but I'm not sure how hard it would be to rework your
code, or if there are any downsides of doing this.

This was what my previous version did: https://lkml.org/lkml/2020/6/7/28.

This approach was not welcomed very well and it fixed only the problem
of the implementation of relocatable kernel. The second issue I'm trying
to resolve here is to support both 3 and 4 level page tables using the
same kernel without being relocatable (which would introduce performance
penalty). I can't do it when the kernel mapping is in the vmalloc region
since vmalloc region relies on PAGE_OFFSET which is different on both 3
and 4 level page table and that would then require the kernel to be
relocatable.

Ok, I see.

I suppose it might work if you moved the direct-map to the lowest
address and the vmalloc area (incorporating the kernel mapping,
modules, pio, and fixmap at fixed addresses) to the very top of the
address space, but you probably already considered and rejected
that for other reasons.

         Arnd